In the field of producing various kinds of electronic devices requiring fine working such as a semiconductor element, the demand for increasing the density and the integration of devices has been increased more and more. With the demand, the required performance for the photographic techniques for realizing fining of patterns becomes extremely severe. The technique of playing a part of the fining techniques is the increase of the high resolving power of a photoresist and also is shortening the wavelength of the exposure light.
In general, the resolution (Res) in an optical system can be expressed by the Rayleigh's equation, that is Res=k.multidot..lambda./NA (wherein k is a process factor, .lambda. is a wavelength of the exposure light source, and NA is a numerical aperture of a lens). From the equation, it can be seen that for more fining the reproduced line width and resolving fine patterns (that is, for obtaining a high resolving power), the wavelength at exposure may be shortened. Certainly, with the reduction of the smallest reproduced line width, the exposure wavelength is shifted to the g ray (436 nm) and the i ray (365 nm) of a high-pressure mercury lamp, and further, the production of devices using a KrF excimer laser (248 nm) has been investigated. Also, for finer working, the utilization of shorter excimer laser, in particular, ArF excimer laser (193 nm) has been promising.
On the other hand, when a photoresist which is exposed with the light of a short wavelength is watched, the increase of the high integration by not a single layer resist conventionally produced but by double layer or more multilayer resist system utilizing a surface lithography has been investigated. However, there is yet a problem about the complication of process which has hitherto hindered the practical use of a multilayer resist.
Also, in the case of an excimer laser such as a KrF excimer laser, because the life of the gas is short and the exposure apparatus itself is expensive, it is generally considered to be necessary to increase the cost performance of laser.
In response to the necessity, there is a so-called chemical amplification-type resist which becomes the main current from the use for the KrF excimer laser exposure. The chemical amplification resist has a mechanism that an acid is generated from a photo acid generator existing in the system in a catalytic amount by a light exposure, by the catalytic amount of the acid, the protective group of an alkali-soluble group of a binder or a low molecular compound is released as in catalytic reaction and the discrimination of the solubility to an alkali developer is secured. Because the chemical amplification-type resist catalytic reactively utilizes the acid generated by a photoreaction, the increase of the sensitivity can be expected.
The greater part of the photoresists for the excimer laser exposure under development at present are the chemical amplification-type resists and this is also applied to the application for an ArF excimer laser having an exposure wavelength of 220 nm or shorter.
Now, the largest theme in the photoresist system corresponding to a stepper having exposure wavelengths of from 170 to 220 nm aimed at by the present invention is securing of the transparency of the resist film to the wavelength of the exposed light. The quinonediazido series compound which is used in the case of the g ray (436 nm) and the i ray (365 nm) of a high-pressure mercury lamp as the exposure light has a property of reducing the absorption to the wavelength of the exposed light by the light exposure, that is, has a light bleaching property, and thus the absorption by the film before the light exposure does not give a problem as at present. On the other hand, in the case of the chemical amplification-type resist, because the resist does not have such a light bleaching property, when the light absorption before light exposure is high, a large difference of the light intensity occurs between the surface of the film and the bottom portion of the resist and as the result thereof, a high resolving property is not obtained. Thus, in the case of the chemical amplification-type resist, the transparency of the resist film before light exposure is regarded as important.
In the case of planning a transparent material to a light having a wavelength of 220 nm or shorter, first, it is said that the use of aromatic compounds conventionally used as materials for positive resists is difficult. For example, even novolac resins and further polyhydroxystyrene which are conventionally frequently used as binders have a high absorption to the light of the wavelength of 220 nm or lower. Accordingly, for a lithography utilizing the exposing light wavelength of 220 nm or shorter, these materials cannot be used.
The above thing is also applied to the other material, the photo acid generator. For example, in SPIE Proceedings, pages 422-431, Vol.2438 (1995), it is reported that the added amount of the triphenylsulfonium salt-type light acid generating agent, which is frequently used in the resists for the KrF excimer laser, is limited because of a large light absorption in a vacuum ultraviolet region. On the other hand, it is reported that among the aromatic series compounds, naphthalene has a very small light absorption to the ArF excimer laser (exposure wavelength: 193 nm), e.g., in J. Photopolym. Sci. Technol., pages 423-432, Vol. 7 (1994), and SPIE Proceedings, pages 445-454, Vol. 2438 (1995).
That is, in the aromatic series materials which are generally considered to be unusable in the lithography utilizing the light for exposure having a wavelength of 220 nm or shorter, it can be said that usable materials exist.
Also, in regard to the photo acid generators not having an aromatic substituent in the molecule and the small absorbance to the light having a wavelength of 220 nm or shorter, for example, the photo acid generator represented by following formula is reported in JP-A-7-199467, JP-A-7-252324, and JP-A-8-12626 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application"); ##STR1##
However, the photo acid generator is unsatisfactory on the photodecomposition property, i.e., the acid generating efficiency, which is another important property for the photo acid generator.
As described above, in the lithography utilizing the light for exposure having a wavelength of 220 nm or shorter, such as the ArF excimer laser, systematic investigations of the photo acid generator have not yet been made and thus it is very difficult to anticipate the relation of the structure and the light absorption. Furthermore, it can be said that there are no knowledge about the transparency and photodecomposition property or the acid generating efficiency to an exposed light.
That is, the means of obtaining a photo acid generator having a high transparency to an exposure light, satisfying the photodecomposition property or the acid generating efficiency to the exposure light, and being usable in the lithography utilizing the exposure light having a wavelength of from 170 to 220 nm, which is aimed in the present invention, have hitherto been entirely not known.